It has become apparent that the earth's natural resources are not unlimited and that mankind's ultimate survival depends on proper conservation of those resources, both by controlling the depletion of raw materials and by eliminating pollution of our land, water and atmosphere. At the same time the world-wide desire for an ever increasing standard of living provides a great pressure for rapid production and consumption of goods at the lowest possible immediate cost. Faced with resolving these conflicting demands, planners are increasingly turning to the concept of recycling to provide efficient utilization of raw materials, reduce the output of pollutants and maintain an acceptable supply of goods for our economy.
One class of materials which are prime candidates for recycling, is the non-ferrous metals, particularly aluminum. Aluminum accounts for more than 95 percent of the non-ferrous metal content of municipal waste in most localities. The resistance of aluminum food and beverage containers and other aluminum articles to degradation makes them particularly obnoxious pollutants because of their relative permanency. Further, the high economic value of aluminum coupled with the vast amounts of energy which must be expended to obtain aluminum metal from natural ores, make the recycling of aluminum economically attractive.
The principal obstacle in recycling non-ferrous metals such as aluminum is the difficulty of separating them from other waste materials. The usual route of discarded aluminum food and beverage containers and other aluminum objects is through municipal waste systems which yield a thoroughly co-mingled mixture of materials of staggering diversity.
Electrostatic separation has been practiced for years, primarily in the treatment of grains and mineral ores. Actually the use of the term "electrostatic" is a misnomer because most so-called "electrostatic separators" leak an appreciable current between electrodes and thus are dynamic instead of static in operation. Consequently, the term "high tension separation" is to be preferred and will be utilized throughout the remainder of this specification. Efficient operation of high tension separations has generally required reduction of ore particles to small size because only very narrow size ranges and low mass particles could be effectively separated. Although mineral ores are not usually considered homogeneous mixtures, they are much more homogeneous than municipal waste, which incorporates the most diverse types of materials conceivable and is probably the most heterogeneous mixture known. The very heterogeneous nature of municipal waste coupled with the vast bulk of the material and the comparatively low economic value of the majority of the constituents has made it impractical to attempt to reduce the waste to small and relatively uniform particle sizes of the magnitude thought essential for efficient high tension separations. Thus, prior efforts to apply high tension separation techniques to the treatment of municipal waste have not been successful.
A further problem which arises in attempts to separate non-ferrous metals from municipal waste by high tension separation methods, is the inclusion of large amounts of lower conductivity conductive materials in the collected non-ferrous metal material. The expression low conductivity conductive materials is intended to refer to materials having a resistivity greater than about 10.sup.-.sup.2 ohms per cm. at 20.degree.C. High tension separation methods are most effective in separating good conductors from substantial non-conductors, e.g., in separating the conductive metal wire from the non-conductive insulation in shredded electrical wire scrap. High tension separation techniques are much less selective in separating low conductivity conductive materials from other conductive materials such as the non-ferrous metals. Particular difficulty is encountered in attempting to separate aluminum from waste streams containinng appreciable amounts of damp or wet wood which has a lower, but nevertheless appreciable conductivity. Accordingly, the separation of non-ferrous metals from municipal waste streams which usually include more low conductivity conductive constituents such as damp wood than they do highly conductive non-ferrous metal constituents such as aluminum, presents special problems.
It is an object of this invention to provide an apparatus and a method for separating non-ferrous metal from municipal waste.
It is a further object of this invention to provide an apparatus and method capable of operating efficiently on feed material having particle sizes substantially larger than previously processed by high tension methods.
It is a further object of this invention to provide an apparatus and method for separating non-ferrous metal from waste material which can produce material of high non-ferrous metal content from mixtures containing amounts of non-ferrous metal and substantial amounts of low conductivity conductive materials.
It is also an object of this invention to provide a method for separating non-ferrous metals from waste material which can be conveniently added onto existing waste processing installations with little or no modification of the rest of the ongoing process.
Further objects of this invention will be apparent from a consideration of the specification.